The present disclosure relates to the use of ink containing luminescent dyes for machine readability and/or security applications.
There is a need for many products to include a machine readable code or mark. Existing online printing systems are either too costly to implement or are too unreliable due to several key aspects. First, the machine readable or security code is applied at high speed directly onto, for example, mail envelopes or the packaging material of consumer goods and then needs to be subsequently read with high precision and repeatability to be verified online by the system. Camera-based reading systems are often employed in the verification of such codes because they are relatively inexpensive and can be easily adapted to read various code symbologies. As the case with any digitally interpreted image, cameras require a certain dwell time (or acquisition time) over the printed code when attempting to acquire the digital image that is used to decipher and analyze the code therein. The printing background may be highly variable (e.g., including multiple colors) with both light and dark regions. The dark and light regions interfere with what would otherwise be a coherent light pattern emitted from the surface. For example, the light regions artificially amplify the light that the camera sees and, conversely, light is attenuated by the dark regions. For these reasons, it is often difficult, especially at high speed, for an automated system to decipher these codes.
Printed codes from luminescent inks are illuminated by an excitation light source with a given wavelength range and spontaneously emit light at a second wavelength range. The fact that light is actively emitted from the surface and travels directly back to the camera allows for a greater imaging contrast between the printed code and the background as when compared with, for example, a merely light absorbing mark, especially at rapid acquisition rates. For example on a high speed production line (i.e., up to about 1000 feet per minute) an item may spend a small amount of time within a camera's field of focus. There is generally a need for bright luminescent inks that solve these high speed machine readable applications.
For machine readable applications and particularly security applications, there is often a need for the print codes to be durable and to survive environmental conditions for a predetermined life-span after printing. Fluorescent or phosphorescent pigmented inks are often selected for these applications based on their insolubility, water and lightfastness.
There are downsides with to using luminescent pigments instead of dyes for machine readable applications. For example, fluorescent pigments generally exhibit reduced fluorescence as compared to fluorescent dyes. Reduced fluorescent intensity reduces the reader's ability to verify the printed code on a high speed production line. Current approaches that rely on pigments are also less reliable due to problems associated with running pigmented inks reliably in inkjet printers. The inks and systems of this invention lend toward better system uptime, good print quality, verifiability at high speed with camera systems, good water resistance and relatively good light stability.
The needs as outlined above apply to several specific applications including black market security applications, grey market or diversion prevention, producer authentication, postal tracking, transactional printing, and other applications. For all of these applications, it is commonly required that the system exhibit system reliability, good code readability and durability.